1. Field of the Invention
The present invention relates to a disc-shaped record medium wherein information signals are recorded in the information recording areas thereof formed of tracks which are of concentrical or spiral shape.
2. Description of the Prior Art
Hitherto, various recording formats for information recording operation have been conventionally proposed, a case in point being the one called a CAV (Constant Angular Velocity) mode which is a method for keeping constant the revolution frequency of a spindle and frequency for recording or reproducing information on or from a record medium.
However, there is a defect in the record medium in which information is recorded in the above CAV mode such that the information recording capacity thereof as a whole is reduced since the recording density gets reduced in proportion to the distance from the center portion of the record medium, and in order to eliminate this problem, there has been provided a ZCAV (Zone Constant Angular Velocity) mode by which recording capacity of the record medium as a whole can be increased taking advantage of an easy revolution control of the CAV mode. In other words, the ZCAV mode is a method in which a revolution frequency of the spindle is kept constant during the whole recording or reproducing operation, whereas the frequency for recording or reproducing information on or from the record medium is increased step by step in proportion to the distance from the center of the record medium. By the way, although it is common to set a track pitch (a distance between two adjacent tracks) normally to 1.5 to 1.6 .mu.m, there has also been proposed an improved CAV mode in which information recording capacity of the record medium is increased by making the track pitches narrower in proportion to the distance from the center of the record medium.
It is already known that the recording capacity of the record medium in the above explained ZCAV mode can be further increased by making the track pitches within a record medium variable. However in recent years, it is still required in the modern society to provide a method by which a large quantity of various information can be recorded or reproduced on or from a record medium of a smaller size so as to further increase the recording density thereof.